1. Field of the Invention
The present invention relates to a hydraulic power boost mechanism for an automotive vehicle such as a truck, forklift truck or passenger car. More particularly, the present invention relates to an improvement for increasing the boosting pressure of the boost mechanism for a braking system of an automotive vehicle.
2. Description of the Prior Art
As is well-known to those skilled in the art, a hydraulic boost mechanism, which will be hereinafter referred to as a power booster or hydraulic power booster, is provided between a foot pedal and a split system master cylinder for application of boost force to a braking system of an automotive vehicle when the foot pedal is moved into the braking position. Such a hydraulic power booster has been provided in the braking system and has been coupled with a steering gear system for sharing hydraulic power therewith.
Generally the hydraulic power booster has a housing in which a piston reciprocates; the piston divides the housing into an exhaust chamber and a pressure chamber. The piston is provided with a longitudinal bore which receives a control valve means including a spool valve means which can be moved to a released position communicating the pressure chamber to the exhaust chamber and also communicating an inlet port to an outlet port which is connected to a steering gear device, and to an applied position connecting the pressure chamber to a hydraulic fluid pressure source for developing a boost pressure in the pressure chamber and thus for driving the piston. The booster further has a means for co-operating the foot pedal with the piston in order to move the spool valve means of the piston into the applied position when the foot pedal is moved to the braking position. Then, the spool valve means narrows and finally closes the channel communicating the inlet port to the outlet port and also narrows the channel communicating the pressure chamber to the exhaust chamber. In turn, the spool valve means opens the channel communicating the inlet port to the pressure chamber. Thereby, hydraulic fluid flowing into the booster is applied to the pressure chamber to develop the boost pressure for driving the piston for application of the master cylinder so as to decelerate the vehicle and thus for applying a hydraulic reaction for stiffening the pedal.
In a conventional booster, since the thrust plunger as the means for co-operating the foot pedal with the powering piston has been integrally formed, the ratio of hydraulic boost force applied to the piston for driving the same and hydraulic reaction for the foot pedal is fixedly determined as a function of the ratio of the diameters of the plunger and the piston. When a need to change the ratio arises, it is necessary to change the diameter of the plunger relative to that of the piston. It will be noted that, to render it fluid proof the plunger is received slidably but has a minimum clearance with the housing. Therefore, to change the diameter of the plunger may involve changing the diameter of the bore and thereby changing the housing. In practice, for varying the ratio of boost force and hydraulic reaction, it has been required to use a plurality of power boosters having various ratios of the boost force and hydraulic reaction. This causes complications of construction of a hydraulic braking system as well as produces a high cost for the system.
Further, it is desired that the ratio of hydraulic pressure applied to the piston for driving the same relative to the hydraulic reaction acting on the foot pedal should as great as possible. The above mentioned ratio is calculated by the following formula: EQU R=A.sub.2 /A.sub.3
where:
R is the ratio of boost force to hydraulic reaction; PA1 A.sub.2 is a cross-sectional area of the power piston, at a portion where the boost pressure is applied; PA1 A.sub.3 is a cross-sectional area of the head of a thrust plunger as a connecting means for connecting the piston to the foot pedal. PA1 ( a) increasing the sectional area of the piston naturally requires increasing the sectional area of the bore formed in the booster housing resulting in an increase in the volume of pressure fluid needed for applying the booster; PA1 ( b) increasing the fluid volume may cause an increased loss of energy; PA1 ( c) since such a booster can apply greater pressure with smaller operating power, when relief pressure is applied to the steering system, too much pressure may possibly be applied to satisfactorily match the combination of steering system and the boost system.
It will be obvious that increasing the ratio of boost force can be accomplished by making A.sub.3 smaller or making A.sub.2 larger. However, since the head of the thrust plunger is subject both to the force moving the foot pedal into the braking position and the hydraulic reaction due to the boost pressure, it is in practice difficult to decrease the sectional area thereof. Thereby, it has been required for increasing the ratio of boost force to increase the sectional area of the piston. Increasing the sectional area of the piston may cause various problems, such as:
Therefore, in practice, the conventional booster has been limited to a pressure ratio on the order of 6:1.